The Mexico core is vital to this PPG; it is very efficiently directed by Drs. Ruiz-Palacios and Maria de Lourdes Guerrero. Dr. Guerrero is closely connected to all aspects of the core, she personally knows and counsels each of the mothers in this project, is knowledgeable about health conditions and all other aspects of each participant's home at all times. The core will continue to provide important information during the current transition of the San Pedro Martir area of Mexico City from semi-rural area where most residents were poor to a more urbanized and diversified area that includes some middle class families. The 316 mother-infant pairs recruited into the study during the 1988-1992 period have provided the specimens that formed and continue to form the backbone of this PPG. A new cohort of 306 mother-infant pairs was enrolled as of April 2001. Specimen collection is exemplary in this group with complete documentation of health status for all mother infant pairs enrolled earlier (1988-1992) and currently (up to 2001). All projects of the PPG depend on these specimens which so far have provided the most significant insight into the specific role of non-immune protection of the breast-fed infant and young child by mother's milk. These studies have also been able to compare the specific (but more limited) protection of the infant by immune (milk IgA) with that of the non-immune protection provided by milk oligosaccharides and lactoferrin. The Mexico site is also important for the clinical trials conducted as part of the PPG activities.